The fraud has caused millions of people to fill their cars with substandard gasoline that may have violated clean air standards, or to drink water not properly tested for safety, officials told the Associated Press.

In addition, officials making decisions at hazardous-waste cleanup sites have relied on companies that fraudulently tested air, water and soil samples.

"In recent years, what has come to our attention is that [nongovernment] labs are oftentimes in bed with the people who hired them, and conspired to commit environmental crime," said David Uhlmann, chief of the Justice Department's environmental crimes section.

The EPA's watchdog against fraud, Inspector General Nikki Tinsley, has called the rise of lab fraud a disturbing trend.

"If it was my drinking water, I'd consider it very serious," she said, declining to identify locations affected by the ongoing investigation.

Private laboratories test products regulated by anti-pollution laws, and the results allow companies to certify that they are meeting such laws' requirements.

In one instance three years ago, investigators discovered fraudulent test results by contract employees at the Environmental Protection Agency lab in Chicago. The head of the laboratory was transferred and the contractor, Lockheed Martin, was suspended from performing tests.

The Justice Department and Environmental Protection Agency have prosecuted dozens of employees and laboratories in the past several years for fraudulent testing. Mr. Uhlmann said the prosecutions have grown but statistics are not kept on lab-fraud cases.

The growing number of cases stretches from New England, where a chemist for municipal water made up test results, to Texas, where the government recently prosecuted the largest tester of underground fuel tanks.

Officials said they aren't certain whether more labs are falsifying tests, or whether more are simply being caught through more-aggressive investigations and whistleblowers.

Sometimes the fraud includes "driveway tests," so named because employees generate them on computers in their own driveways, without ever visiting the facilities.

Whatever the case, lab fraud hampers an environmental-protection system that frequently relies on voluntary compliance by companies backed by test results, officials said. Faked results can mislead regulators and the public into thinking they are being protected by laws when in fact companies are not abiding by the safeguards.